Various electronic parts are usually installed at a pre-determined position on a copper wiring pattern formed on a print substrate, and soldered to form an electronic circuit wiring. Conventional electronic circuits had electronic devices bonded on a print substrate made of paper phenol resin, epoxy resin, glass epoxy resin, or others using a solder layer containing Pb (lead) as a main component.
However, solders containing lead tend to be restricted from use in recent years for reducing environmental load. Especially in Europe, their use has been banned since Jul. 1, 2006 by the RoHS Directive (directive of the European Assembly and Directorate on restriction of the use of certain hazardous substances in electrical and electronic equipments). Consequently, lead free solders such as those formed of Au—Sn, Ag—Sn, In—Sn, Zn—Sn, and Bi—Sn have been proposed as alternatives of the lead and tin containing solder.
Nonpatent Reference 1 reports that, when a solder not containing lead is left at room temperature, it takes more than 10 months for equilibrium to be reached. Nonpatent Reference 2 reports that, when a load by a heat cycle is applied after solder bonding in a semiconductor laser package using a solder not containing lead, the strength of the solder bonding changes gradually due to said heat cycle load.
Nonpatent Reference 3 discloses a method to bond an electronic device to a substrate by contacting the device to a solder layer from the start of heating of the solder layer, pressing the device to the solder layer, and changing its pressure by the solder layer temperature. FIG. 17 is a phase diagram (See Nonpatent Reference 4.) illustrating the phases of an alloy consisting of Au and Sn (Au—Sn alloy) used in Nonpatent Reference 3 by atomic ratio.    [Nonpatent Reference 1] V. SIMIC and Z. MARINKOVIC, “Thin film interdiffusion of Au and Sn at room temperature”, J. Less-Common Metals, 51, pp. 177-179, 1977    [Nonpatent Reference 2] J-H. Kuang and 5 others, “Effect of Temperature Cycling on Joint Strength of PbSn and AuSn Solders in Laser Packages”, IEEE Trans., Adv. Pack, Vol. 24, No. 4, pp. 563-568, 2001    [Nonpatent Reference 3] Kenji YAMAGUCHI and 3 others, “Study of Au/Sn Bonding in Manufacture of Interpitch Multilayered Lead Frame (1)” (in Japanese), Abstracts of National Meeting of Japan Welding Society, Vol. 49, pp. 410-411, September, 1991    [Nonpatent Reference 4] “Metal Databook”, edited by The Japan Institute of Metals, Third Edition, Maruzen, Mar. 25, 1993, pp. 410    [Nonpatent Reference 5] O. Kubachewski et. al., “Materials Thermochemistry”, 6th Edition, Pergamon Press, pp. 258-323 (Table 1), 1993    [Nonpatent Reference 6] O. Kubachewski et. al., “Metallurgical Thermochemistry”, 5th Edition, Pergamon Press, pp. 268-323 (Table A), 1979